Children are placed into 1 of 4 treatment groups based on age: Group 1, 3 months to 24 months; Group 2, 25 months to 6 years; Group 3, 7 years to 12 years; Group 4, 13 years to 17 years. They receive baseline evaluations. Antiretroviral-naive patients receive emtricitabine plus stavudine plus lopinavir/ritonavir. Antiretroviral-experienced patients replace lamivudine with emtricitabine. Patients return to the clinic for follow-up visits at Weeks 2 and 4 and then every 4 weeks until Week 48. Safety is evaluated using adverse events, which are reviewed at every clinic visit. Clinical laboratory data and full-profile pharmacokinetics of emtricitabine are evaluated at some visits. After the Week 48 study evaluations are completed, individual patients may continue to receive study medication (as provided by the sponsor) until commercially available, if certain criteria are met.